On Mon, 24 Oct 2022, Andre Vieira (lists) wrote:
>
> On 24/10/2022 08:17, Richard Biener wrote:
> >
> > Can you check why vect_find_stmt_data_reference doesn't trip on the
> >
> > if (TREE_CODE (DR_REF (dr)) == COMPONENT_REF
> > && DECL_BIT_FIELD (TREE_OPERAND (DR_REF (dr), 1)))
> > {
> > free_data_ref (dr);
> > return opt_result::failure_at (stmt,
> > "not vectorized:"
> > " statement is an unsupported"
> > " bitfield access %G", stmt);
> > }
>
> It used to, which is why this test didn't trigger the error before my patch,
> but we lower it to BIT_FIELD_REFs in ifcvt now so it is no longer a
> DECL_BIT_FIELD.
>
> But that is a red-herring, if you change the test structure's 'type Int24 is
> mod 2**24;' to 'type Int24 is mod 2**32;', thus making the field we access a
> normal 32-bit integer, the field no longer is a DECL_BIT_FIELD and thus my
> lowering does nothing. However, you will still get the failure because the
> field before it is a packed 4-bit field, making the offset to the field we are
> accessing less than BITS_PER_UNIT.
Hmm, so the _intent_ of DECL_BIT_FIELD_REPRESENTATIVE is to definitely
_not_ be a DECL_BIT_FIELD (well, that's the whole point!). So this
shows an issue with setting up DECL_BIT_FIELD_REPRESENTATIVE? Of course
for a type with an alignment less than BITS_PER_UNIT (is StructB actually
such a type?) there cannot be a representative that isn't, so maybe
we should then set DECL_BIT_FIELD on it with a condition like Eric
mentions?
> > ? I think we should amend this check and I guess that
> > checking multiple_p on DECL_FIELD_BIT_OFFSET should be enough?
> That won't work either, unless we do the same walk-through the full access as
> we do in get_inner_reference.
I suppose we should not "if-convert" bit field accesses with a
DECL_BIT_FIELD representative. There isn't any benefit doing that
(not for general bitfield lowering either).
Richard.
> Let me elaborate, the 'offending' stmt here is:
> _ifc__23 = (*x_7(D))[_1].b.D.3707;
>
> And the struct in question is:
> package Loop_Optimization23_Pkg is
> type Nibble is mod 2**4;
> type Int24 is mod 2**24;
> type StructA is record
> a : Nibble;
> b : Int24;
> end record;
> pragma Pack(StructA);
> type StructB is record
> a : Nibble;
> b : StructA;
> end record;
> pragma Pack(StructB);
> type ArrayOfStructB is array(0..100) of StructB;
> procedure Foo (X : in out ArrayOfStructB);
> end Loop_Optimization23_Pkg;
>
> That D.3707 is the 'container'm i.e. the DECL_BIT_FIELD_REPRESENTATIVE of the
> original bitfield of type Int24.
> So in vect_find_stmt_data_reference , the dr is: (*x_7(D))[_1].b.D.3707 and
> TREE_OPERAND (DR_REF (dr), 1): D.3707,
> which has DECL_FIELD_BIT_OFFSET: 0
>
> So that check would also pass. However, get_inner_reference, walks the full
> access and comes across '.b', the member access for StructA inside StructB,
> that has DECL_FIELD_BIT_OFFSET: 4
> Which is where we get into trouble. So to catch that here, we would need to do
> the same type of walking through all the member accesses, like
> get_inner_reference does.
>
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)
